1. Field of the Invention
The present invention relates to a variable gain amplifier. More particularly, the present invention relates to a variable gain amplifier in which a gain changes in an exponential function manner according to a gain control voltage or a gain control current.
2. Description of the Related Art
In recent years, development of a CDMA (Code Division Multiple Access) system with a good use efficiency of a frequency is active underway. In the CDMA system, the range of control is wide with respect to a transmission power control and fine control is required. Thus, there is a need for a variable gain amplifier with high precision capable of achieving a wide range and fine transmission power control.
FIG. 14 shows an example of a conventional variable gain amplifier using a Gilbert multiplier. Here, a differential type variable gain amplifier is shown as an example. This amplifier has a feature that degradation of control precision due to distortion of element features is small.
That is, this variable gain amplifier has NPN transistors Qa, Qb which configure a differential transistor pair and NPN transistors Qc, Qd which configure a differential transistor pair. Base terminals of the transistor Qa and the transistor Qc each are connected in common so that a gain control voltage VY is supplied between this common base terminal and each base terminal of the transistor Qb and the transistor Qd.
A collector terminal of an NPN transistor Q101 is connected to a common emitter terminal of the transistors Qa, Qb. An emitter terminal of the transistor Q101 is connected to a ground potential via a voltage/current converter 102. A collector terminal of an NPN transistor Q103 is connected to a common emitter terminal of the transistors Qc, Qd. An emitter terminal of the transistor Q103 is connected to a ground potential via a voltage/current converter 104. Then, an input signal voltage vin is supplied between base terminals of the transistor Q101 and the transistor Q103. The emitter terminal of the transistor Q101 and the emitter terminal of the transistor Q103 are connected in common via a resistor RE.
On the other hand, a power voltage from a power line VL is supplied to a collector terminal of the transistor Qb. To a collector terminal of the transistor Qa, the power voltage from the power line VL is supplied via a resistor R1. The power voltage from the power line VL is supplied to a collector terminal of the transistor Qd. To a collector terminal of the transistor Qc, the power voltage from the power line VL is supplied via a resistor R2. Then, an output signal voltage vout according to a current ic is taken out from the collector terminals of the transistors Qa, Qc.
In the case of the amplifier shown in FIG. 14, a voltage gain GY relevant to the gain control voltage VY can be expressed as shown in the following formula (1). In the formula, A is a constant, and VT is a thermal voltage. exp represents an exponential function.GV=A/(1+exp(VY/VT)  (1)
In the above formula (1), “1” of denominator can be ignored in a range such that exp(VY/VT)>>1. Therefore, the above voltage gain GV can be rewritten as shown in the following formula (2).GV=A·exp(−VY/VT)  (2)
In general, in order to make gain control with high precision, it is desired that a relationship between the gain control voltage VY and the voltage gain GV be log-linear. In a range of exp(VY/VT)>>1, in a conventional variable gain amplifier as well, a log-linear gain change can be achieved relevant to the gain control voltage VY. However, in a range of exp(VY/VT)>>1, the voltage gain GV is considerably small, and thus, it is disadvantageous in current consumption and noise. As in a CDMA system, there is a need for an amplifier with a wider, log-linear control range in use such that gain control with high precision is required over a wide range.
In order to achieve a log-linear gain control feature with high precision over a wide range, “1” of a denominator of the above formula (1) must be canceled. For that purpose, the following formula (3) may be established. However, K (>0) is a proportional constant, and VAGC (≧0) is a new gain control voltage of the variable gain amplifier. In addition, “ln” designates a natural logarithm function.VY=VT·ln{exp(K·VAGC/VT)−1}  (3)
At this time, when the above formula (3) is substituted for the above formula (1), the following formula (4) is established. That is, the gain control voltage VAGC and the voltage gain GV are log-linear up to a maximum gain value.GV=A/{exp(K·VAGC/VT)}  (4)
In a variable gain amplifier shown in FIG. 15, a converter (VAGC→VT) 105 of a gain control voltage is provided to achieve the above formula (3). That is, this converter 105 has NPN transistors Qe, Qf which configure a differential transistor pair. A base terminal and a connector terminal of the transistor Qe is connected to the common base terminal of the transistors Qa, Qc. A base terminal of the transistor Qf is connected to the base terminals of the transistor Qb and the transistor Qd each. Then, the gain control voltage VY is supplied between the common base terminal of the transistor Qa and the transistor Qc and each of the base terminals of the transistor Qb and the transistor Qd. In addition, a reference bias voltage VBIAS is supplied between a base terminal of the transistor Qf and each of the base terminals of the transistors Qb, Qd. The power voltage from the power line VL is supplied to a collector terminal of the transistor Qf.
Moreover, a collector terminal of one PNP transistor Qh which configures a current mirror circuit is connected to the base terminal and collector terminal of the transistor Qe. To an emitter terminal of the transistor Qh, the power voltage of the power line VL is supplied via a resistor R4. A base terminal of the transistor Qh is connected to a base terminal and a collector terminal of the other PNP transistor Qg which configures the current mirror circuit. To an emitter terminal of the transistor Qg, the power voltage from the power line VL is supplied via a resistor R3. A collector terminal of an NPN transistor Qi is connected to the base terminal and collector terminal of the transistor Qg. An emitter terminal of the transistor Qi is connected to a ground potential. A base terminal of the transistor Qi is connected to a ground potential via a voltage/current converter 106, and connected to a base terminal of an NPN transistor Qj. In addition, a reference base voltage VB is supplied to a base terminal of the transistor Qj. A collector terminal of the transistor Qj is connected to a common emitter terminal of the transistors Qe, Qf. An emitter terminal of the transistor Qj is connected to a ground potential.
In the case of the variable gain amplifier, the gain control voltage VAGC is supplied to the base terminal of the transistor Qi via the voltage/current converter 106. In this manner, a gain control current IAGC proportional to the gain control voltage VAGC is provided to the resistor R. Namely, a voltage proportional to the gain control voltage VAGC is provided to the resistor R.
Here, assuming that I0 is a constant current, a current I1 (≧0) which is I0·exp(−K·VAGC/VT) flows the transistor Qe according to the gain control voltage VAGC. This current I1 generates a voltage VBEQe between a base and an emitter to the transistor Qe. On the other hand, a current I2 (=I0−I1) flows the transistor Qf, and generates a voltage VBEQf between a base and an emitter.
A differential VY (=VBEQf−VBEQe) between these voltages is as shown in the following formula (6) and meets a relationship of the above formula (3).                                                                         V                Y                            =                            ⁢                                                                    V                    T                                    ·                                      ln                    ⁡                                          [                                                                        I                          0                                                ⁢                                                  {                                                      1                            -                                                          exp                              ⁡                                                              (                                                                                                      -                                    K                                                                    ·                                                                                                            V                                      AGC                                                                        /                                                                          V                                      T                                                                                                                                      )                                                                                                              }                                                                    ]                                                                      -                                                                                                      ⁢                                                                    V                    T                                    ·                  ln                                ⁢                                  {                                                            I                      0                                        ·                                          exp                      ⁡                                              (                                                                              -                            K                                                    ·                                                                                    V                              AGC                                                        /                                                          V                              T                                                                                                      )                                                                              }                                                                                        (        5        )                                                           ⁢                  =                    ⁢                                                    V                T                            ·              ln                        ⁢                          {                                                exp                  ⁡                                      (                                          K                      ·                                                                        V                          AGC                                                /                                                  V                          T                                                                                      )                                                  -                1                            }                                                          (        6        )            
Therefore, in the case of the variable control amplifier shown in FIG. 15, a log-linear gain control feature with high precision over a wide range can be achieved (reference should be made to U.S. Pat. No. 6,215,989 B1, for example, for details).
FIG. 16 shows voltage gain features of the variable gain amplifier shown in FIG. 14 and the variable gain amplifier shown in FIG. 15 for comparison. In FIG. 16, gain control voltages VY, VAGC on a horizontal axis are standardized. As is evident from FIG. 16, it is found that the variable gain amplifier of FIG. 15 has more log-linear gain control feature over a very wide range (about 60 dB) than the variable gain amplifier of FIG. 14.
As described above, in the conventional variable gain amplifier, there can be achieved an exponential function shaped gain control feature in which a gain changes relevant to an input of a linear gain control voltage (VAGC) in an exponential function manner, namely, an exponential gain control feature G=1/exp(K·VAGC/VT) can be achieved. However, there has been a problem that as the gain increases, a dynamic range decreases, and a distortion feature is likely to be impaired. That is, in a negative half-period of an output voltage waveform, a voltage VCEQa between a collector and an emitter of transistor Qa connected to a load (resistor R1) is reduced. In particular, in the case where an amplitude of the output signal voltage vout is large, the voltage VCEQa between the collector and the emitter is smaller than a saturation voltage between the collector and the emitter (for example, 0.3V). In this manner, the transistor Qa enters a saturation state, and a distortion of an output voltage waveform rapidly increases.
In this manner, although a base voltage of the transistor Qb is maintained to be constant by the reference bias voltage VBIAS, the base voltage of the transistor Qa fluctuates according to a gain, and is higher as the gain increases. Thus, as the gain increases, the dynamic range is reduced, and the distortion feature is likely to be impaired.